1. Field of Invention
The present invention relates generally to an angular position sensing and indicating system, and more specifically, to a system which includes a contactless arrangement for sensing the angle of rotation of a rotary shaft through 360 degrees of rotation, utilizing magnetoresistive sensors operating in a non-saturating magnetic field, and which is adapted to produce conditioned periodic analog output signals suitable for directly driving a remote electromagnetic shaft position indicator or a relatively simple electronic position indicating apparatus through the full 360 degrees of shaft rotation.
2. Description of the Prior Art
Angular position sensing for 360-degrees of rotation has historically been accomplished using potentiometers, synchros, or resolvers which include low reliability electrical contact arrangements such as electrical brushes and wipers. These devices typically interface with remote position indicating devices which also include potentiometers, synchros, and resolvers.
The need for higher reliability shaft angle sensing for applications such as aircraft control surfaces and closed loop actuators has led to the application of rotary variable differential transformers and absolute encoders which do not rely on a low reliability electrical contact arrangement. Unfortunately, however, these sensors are substantially more expensive and require sophisticated and expensive demodulation electronics to obtain useable output signals.
As a result, efforts to achieve a lower-cost, yet reliable and accurate alternate apparatus for sensing angular position of a rotary shaft have included attempts to utilize sensors such as Hall effect sensors or magnetoresistive (MR) sensors that are capable of generating an electrical output signal when exposed to a rotating magnet field.
Hall effect sensors utilize a current-carrying semi-conductor membrane to generate a low voltage perpendicular to the direction of current flow when subjected to a magnetic field normal to the surface of the membrane. Most Hall effect arrangements are digital in nature, and are utilized for sensing shaft speed or incremental angular position with the Hall effect devices acting as switches or pulse generators. See e.g., Kajimoto et al., U.S. Pat. No. 5,574,364. Some devices utilize the Hall sensors to provide an analog output which is linear over a limited range. Complicated ferromagnetic structures are required to maximize the linear range, which has been increased to approximately 80 degrees for the embodiment proposed in Luetzow, U.S. Pat. No. 5,444,369. PCT/EP 98/03149, published Dec. 3, 1998, Int. Pub. No. WO 98/54547, discloses an arrangement using four individual Hall effect sensors carefully arranged on diagonals of a small square under a uniform rotating magnetic field to provide unique output signal combinations over 360 degrees of rotation. In this instance, each Hall effect sensor provides an output voltage proportional to magnetic flux which is dependent on magnet strength and distance from the magnet to the sensor, and an approach similar to EP 0 217 478 B1 (discussed below) for determination of the angle of rotation. When diagonally opposite sensors are used to create differential signals as proposed, the individual Hall effect sensor characteristics must be matched with each other, and they must be positioned and oriented very accurately with respect to each other and to the rotating magnetic field. Use of a ferromagnetic yoke in preferred embodiments to align the magnetic field across the sensors results in additional hysteresis in the output signals.
Magnetoresistive sensors utilize an element whose resistance changes in the presence of a changing external magnetic field. In application to sensing of a rotating magnetic field, MR sensors are conventionally operated in a saturating magnetic field. Magnetic saturation suppresses the effects of changes in magnetic field strength and external magnetic interference (at least below the level that would result in development of a non-saturating field on the sensor) to insure that only rotational change in direction of the sensed magnetic field causes a resistance change in the MR measuring element.
For sensing angular position of shaft rotation of approximately 120 degrees or less, the conventional approach, as well known and documented in the technical literature, is to utilize the substantially linear portion of a sinusoidal output curve. Application of MR sensors to an accurate yet relatively simple and low-cost apparatus for sensing 360 degrees of rotation has, however, proved to be more elusive, primarily because of the fact that such sensors are inherently limited to sensing rotation of only 180 degrees before the amplitude of the sinusoidal output signal repeats.
Inspection of the prior art illustrates several methods attempting to develop an MR sensor apparatus adapted to determine the direction of the changing magnetic field for developing a suitable linear relationship between sensor output and angular rotation through 360 degrees. However, such prior art apparatus tends to be relatively complicated, sensitive to manufacturing tolerances or difficulties, and/or of relatively high cost.
Certain prior MR sensor arrangements utilize angularly spaced magnetoresistive sensors to sense alternating magnetic poles on the circumference of a shaft to detect angular position or rotation of the shaft. However, such an approach is relatively complicated and expensive as the systems are digital in nature and typically require signal generation for sensor input and pulse counters to develop a suitable output signal. See e.g., Kajimoto et al., U.S. Pat. No. 5,574,364 and Suzuki et al., U.S. Pat. No. 4,791,366.
Alternate arrangements position MR sensors in a plane spaced from and parallel to a rotating magnetic field, such as generated by a magnet located at the end of the rotary shaft, and in a substantially uniform portion of the magnetic field. EP 0 217 478 B1, granted Jul. 24, 1991, utilizes two sensor elements supplied with sinusoidal inputs, phase shifted by 90 degrees, for generating 90 degree phase shifted sinusoidal output voltages which are then electronically or digitally combined and manipulated to determine the angle of the shaft. Muth et al., U.S. Pat. No. 5,602,471, issued Feb. 11, 1997, suggests use of an electronic arrangement to generate a family of sinusoidal-shaped curves, and to then generate a family of overlapping linear ramp characteristics (extracted from the linear portions of the sinusoidal curves) from which a linear relationship is developed between sensor output and angular position over 360 degrees. Andra et al., U.S. Pat. No. 5,796,249 discloses an arrangement of at least three MR sensors, at least one of which is surrounded by an electromagnetic coil energized with an alternating or pulse current to provide the additional directional information needed for angular position sensing through 360 degrees, and to re-shape the natural sinusoidal output curve for a larger linear operating range. Unfortunately, each of these, as well as other related sensor prior art, require additional sensor elements, the addition of an electromagnetic coil, and/or additional signal generators, drive circuitry and complicated output electronics to overcome the sensor limitation of a unique output signal through only 180 degrees of rotation.
Moreover, prior contactless sensing apparatus, including those mentioned above, are generally adapted to providing position indication signal(s) in the form of an electronic or digital signal, that must be further processed or manipulated with additional components if position indication on a remote electromechanical or electromagnetic device is desired.
Thus, it is apparent that there is a long-felt need for an angular position sensing and indicating system comprising a relatively simple, yet reliable and accurate, contactless sensor arrangement adapted to sense 360 degrees of rotation, and to provide an unambiguous output signal or signals suitable for use in indicating the sensed angular position with a relatively simple, yet reliable and preferably contactless electromagnetic, or electronic position indicating apparatus through the full 360 degrees of rotation.